It is generally known in applications of this type that sealing is achieved by creating contact pressure between two parts, one of which is at least partially made of an elastomer, which constitutes the joint, while the other, made of metal for example, constitutes the closure means.
The desire of the user is that the contact pressure created in order to effect the sealing be greater, in an enduring manner, than a predetermined, minimal amount.
In fact, dynamic joints are turned on and off, and thus are subjected or not subjected to contact pressure. It is therefore important that the sealing be retained despite the multiple times that they are utilized.
In addition, the transition from the state of utilization to the state of non-utilization causes the joint to be subjected to various forces and constraints such as friction, elongation and shearing, etc., which call upon its intrinsic properties, in particular the relaxation properties of the elastomer, its behavior of viscosity and elasticity, its mechanical properties and the change in the these properties as a function of temperature and, consequently, the carrying limitations of the joint (the domains of pressure, of temperature, etc.) which are intimately connected with these properties.
In order to explain the problem more precisely, the operation of a sealing gasket of a butterfly valve will be described in detail in the following.
The objective sought with a butterfly valve is to achieve sealing for a given pressure of fluid. However, the best results are achieved when the forces applied to move the closure means are the weakest, and when the contact pressure does not change over time.
It is well known that, between the beginning of the contact between the closure means and the gasket and the end of the movement of the closure means, there is a gradual penetration of the ridge of the closure means into the gasket and a displacement of a wave of elastomer next to the ridge of the closure means.
At the end of the movement, when the valve is shut, there is a section of elastomer which is subjected to constraints of compression due to the crushing and pressing of the gasket, and to the constraints of shearing due to the displacement and to the elongation of the elastomer, considering the friction coefficient existing between the ridge of the closure means and the gasket, It is clear that the energy or force required provided in order to displace the closure means is all the more significant to the extent that the penetration of the ridge into the gasket is significant and the volume of the elastomer displaced is significant.
For a valve in the closed, resting position, the phenomena of relaxation will occur. Volumes of elastomer will again enter equilibrium on both sides of the ridge of the closure means and the elastomer subjected to the compression of the ridge will flow from one side to the other, causing a reduction in the contact pressure and thus a reduction in the sealing level.
The opening of the valve consists of extracting the ridge of the closure means from the gasket. Due to adhesion and to the friction coefficient between the two parts, the elastomer will begin to be stretched under the effect of the movement of the ridge before sliding and a new wave of elastomer will form next to the ridge in opposition to this movement. Here too, the force to be applied will be all the more significant to the extent that the wave is significant.
These phenomena are very classic and the design of valves of this type take them into account, on the one hand by appropriately dimensioning the handle extensions and the manipulators, and on the other hand by increasing the penetration of the ridge of the closure means into the gasket or by increasing the modulus of elasticity of the elastomer. But these solutions give rise to significant inconveniences. From a commercial perspective, there is an increased cost for the valve. From a technical perspective, there are elevated constraints of shearing in the elastomer with a significant risk of tearing when the conditions of pressure or temperature become severe.
At present, the manufacture of joints makes use of appropriate families of elastomers, the formulas and the techniques of manufacture remaining conventional.
Thus, these parts may be made by using a blend with a rubber base of varying make-up: SBR(Styrene-butadiene rubber), EPDM (terpolymer of ethylene, propylene and a diene with the residual unsaturated portion of the diene in the side chain), EPM(Ethylene-propylene copolymer), butadiene-acrylonitrile, polychloroprene, polyethylene chlorosulfonate, fluorous polymers, etc.
The formulation of these rubbers is conventional, based on reinforcers such as carbon blacking of various grades, plasticizers and vulcanization systems effecting monosulfuric or polysulfuric type bonds or bonds of the carbon-carbon type.
The sealing joints are made by molding (done by the vulcanization of the elastomer substantially simultaneously with the molding); the molding may be done by injection, by compression transfer or by the compression of a blank.